Nondestructive metallographic examination of gas turbine components

ABSTRACT

A method is disclosed for the nondestructive examination of coated gas turbine components after service. An electrolytic polishing technique is used to uniformly remove the coating from a portion of the component. An electrolytic etching process is then used to preferentially remove a portion of the underlying substrate so as to leave the microstructure in relief. A metallographic examination may then be made to determine if the part is suitable for reuse. Apparatus for conducting the previously described method is also prevented.

BACKGROUND OF THE INVENTION

The invention disclosed herein was made in the performance of or under acontract with the Department of the Air Force.

1. Field of the Invention

This invention relates to the field of examination and refurbishment ofgas turbine engine components.

2. Description of the Prior Art

Electrochemical techniques are known for the removal of metal from metalarticles. Such techniques are usually used to produce complex surfacecontours or to remove metal from hard or strong materials which couldnot otherwise readily be shaped. Typical of this prior art is that shownin U.S. Pat. Nos. 3,095,364 and 3,372,099. Electrolytic techniques arealso known for the general production of polished surfaces on metalarticles. This is shown in U.S. Pat. No. 3,326,785. It is also known touse electrolytic techniques for the preparation of samples formetallographic examinations. This is shown, for example, in U.S. Pat.Nos. 2,498,220 and 3,434,956. Finally, it is known to use electrolytictechniques for the investigation of the thickness of surface metalliccoatings as shown in U.S. Pat. No. 2,319,196.

SUMMARY OF THE INVENTION

Electrolytic polishing is used to uniformly remove the protectivesurface coating from gas turbine parts, which have been exposed toservice conditions, to expose the substrate. An electrolytic etchingtechnique is then used to reveal the characteristics of the underlyingsubstrate by preferentially removing at least a portion of themicrostructure. The etched substrate is then examined to determine ifthe part under investigation is suitable for recoating and reuse in agas turbine engine. If the part is determined to be satisfactory forreuse, the remainder of the coating may be stripped, and the blade maybe recoated with a protective coating and reinstalled in a gas turbineengine for further use.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of preferred embodiments thereof as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the relationship between voltage and current in a typicalelectrolytic polishing/etching situation.

FIG. 2 shows one type of apparatus which may be used to practice theprocess or the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein the term "gas turbine engine component" will mean anycoated part of a gas turbine engine, which is exposed to elevatedtemperatures, in particular blades and vanes. Modern gas turbine engineshave a limited useful life between overhauls. This is especially true ofthe high performance engines which may be used in military applications.Overhauls are undertaken either as a matter of routine after aparticular length of service has elapsed or as a result of some failureor problem which becomes apparent in service.

Gas turbine engine components are usually fabricated from nickel basesuperalloys. Representative nickel base superalloys are described in thebook entitled "The Superalloys," edited by Simms and Hagel published in1972 by Wiley Interscience Publications. The metallurgy of suchsuperalloys is now fairly well understood; and for a known superalloycomposition, which has had a particular processing history, it ispossible to determine by metallographic examination the maximum servicetemperature to which the superalloy has been exposed. In particular,when an alloy is exposed to a higher than intended service temperature,changes in the size and distribution of the gamma phase will becomeapparent. For exposures only slightly in excess of the temperaturecapability of the alloy the effect on the microstructure will usually bea coarsening or enlargement of the gamma phase particles. Exposure tosomewhat greater temperatures may produce dissolution of the secondphase; and finally, exposure at temperatures much in excess of thecapability of the alloy may result in localized melting. It is,therefore, possible for one skilled in the art to examine a superalloycomponent, which has been exposed to an elevated temperature, andformulate an accurate estimate of the temperature conditions to whichthe alloy has been exposed and to estimate the future life of thecomponent at a particular temperature before failure occurs. Heretofore,such metallographic examinations have been destructive examinationsrequiring that one or more of the used components from an engine besectioned or cut into pieces for examinations. This procedure has theobvious drawback of sacrificing one or more of the components forexamination and of limiting the possibility for general examination ofall components. Such a procedure might be satisfactory in accessing thepotential life remaining in engine components from an engine which wasundergoing a routine overhaul. However, if localized overheating weresuspected, it would be difficult to be certain that the componentexamined was representative of the overheated components. Thus, it is anobject of this invention to provide an examination technique which willprovide for the nondestructive examination of gas turbine enginecomponents and will permit the examination of any or all of thecomponents in an engine.

In electropolishing and electroetching techniques, the surface to bepolished or etched is placed in a suitable electrolyte and is made ananode with respect to a nonreactive cathode, which is immersed in thesame electrolyte. A voltage is then applied between the anode andcathode causing current to flow and resulting in the removal of materialfrom the anode. FIG. 1 shows a schematic of the behavior which mayusually be obtained for a particular combination of anode electrolyteand temperature conditions. This is a desired behavior. The curve inFIG. 1 is seen to consist of two sloping portions connected by arelatively flat portion which is termed a "plateau." Etching or thepreferential removal of one phase of the material producing a roughenedor etched surface will be observed for operating conditions in the twosloping portions of the curve. Voltage conditions between points A andB, the plateau portion of the curve, will produce electropolishing orthe smooth uniform removal of material from the surface.

The present invention requires that an electrolyte and other conditionsbe provided which produce this type of behavior as a function of thevoltage. While specific electrolytes will be described, this inventiondoes not lie in the specific electrolyte, but is generally applicableand useful in conjunction with many electrolytes which produce the typeof behavior shown in FIG. 1. Briefly, the invention process consists ofproviding the used gas turbine engine component to be evaluated,mechanically removing any insulating oxide which may be present on thesurface, electropolishing the surface of the component to provide thesmooth uniform removal of the protective coating; and finally,electroetching the substrate of the article to reveal itsmicrostructural characteristics.

The method of the invention will be better understood through referenceto FIG. 2, which shows an apparatus for performing the invention. FIG. 2shows a container 1, which contains the electrolyte solution 2, and inwhich the component 3 to be examined is supported so that the portion tobe examined is immersed in the electrolyte. The component 3 is shown asa gas turbine blade. In FIG. 2, there is shown a base element 4, whichsupports a vertical support member 5 which in turn, has attached to it,shaped insulating members 6 having cutouts 7, which closely conform tothe cross section of the blade to be examined. These insulating members6 and associated cutouts 7 serve to accurately locate the component.

The particular method of supporting and locating the component withinthe electrolyte is not critical to the invention. Support means could besupplied external to this solution or only one insulator within thesolution might be used in conjunction with an external supportarrangement. In FIG. 2, there is shown a sheet metal cathode 8 having acutout 9 therein which is uniformly larger than the cross section of theblade. Thus, when the blade is properly located within this cutout,there exists a substantially uniform annular space 10 between thecathode 8 and blade 3. Conductive means 11 and 12 serve to connect thecathode 8 and blade 3 with a conventional power supply 13, which isarranged so that the blade is positive with respect to the cathode. Alsoshown in FIG. 2 is a means for circulating the electrolyte so as toprovide for the removal of the products of the electrolytic reactionwhich removes material from the blade 3 in the area near the cathode 8.In FIG. 2, this electrolyte circulation means is shown as an inlet pipe15 and a pickup pipe 16 each having a series of distribution holestherein which are connected to a circulating pump 17 which in turn ispowered by a suitable motor. This pump serves to provide the electrolyteunder pressure to the inlet tube 15 and remove electrolyte through theoutlet tube 16. The pump 17 may be of standard construction and may haveassociated with it suitable filtration means for removing any solidproducts of the electrolytic reaction. The means used to circulate theelectrolyte is not critical, magnetic stirring means have been used inlaboratory evaluations.

The effect of the apparatus shown in FIG. 2 is to remove the coatingfrom the blade in the uniform band around the blade, with the band beinglocated in close proximity to the cathode. The position of the band maybe varied so to provide assessments of the condition of themicrostructure at different portions of the blade. The conditions of themicrostructure at the leading edge of the blade is of particularinterest, since this is generally the portion of the blade which seesthe highest service temperature.

As previously noted, the process employs two electrolytic steps. In thefirst step, electrolytic polishing is used to uniformly remove theprotective coating. In the second step, electrolytic etching is employedto produce a very fine roughening of the surface of the underlyingsubstrate to permit metallographic examination. The change fromelectrolytical polishing to electrolytic etching is achieved by a changein the applied voltage from the power supply. Once the pre-existingprotective coating has been removed by electropolishing and theunderlying substrate has been roughened by electroetching the part isremoved from the solution, rinsed to remove any traces of electrolyte,and is then examined. Examination may be by either optical microscopy orelectron microscopy. If optical microscopy is employed, the surface ofthe blade may be directly examined under an appropriate microscope. Ifelectron microscopy is to be employed, it is necessary to produce areplica of the surface of the article. This replication process may beany one of several well known techniques. A typical replication processemploys a plastic film which is softened in a solvent and then appliedto the surface to be examined so that the softened plastic adapts to theexact contours of the surface to be examined. The softened film isallowed to dry by evaporation of the solvent and is then removed. Thefilm after removal maintains the surface contour and may be shadowedwith metal vapor in preparation for observation in an electronmicroscope. Sample preparation techniques for surface replication aredescribed in numerous books on electron microscopy. In particular in thebook entitled "Manual of Electron Metallography" published in 1973 byThe American Society for Testing And Materials. Replicas produced bythese methods may also be evaluated by optical microscopy and suchreplicas may be stored for future reference.

The previously described process may be better understood throughreference to the following illustrative example. Cast gas turbine enginecomponents consisting of an alloy known as Mar M200+H_(f) having anominal composition of 9 Cr, 10 Co, 12 W, 1 Cb, 2 Ti, 5 Al, 2 H_(f),0.015 B, 0.15 C, which are coated with a protective diffusedaluminum-silicon coating known as PWA 47 having a composition of about90% Al, 10% Si are removed from a gas turbine engine. It is desired todetermine if these blades may be recoated and reinstalled in the engine.A solution consisting of 94% acetic acid and 6% perchloric acid isemployed as an electrolyte. An electrode is provided which surrounds theblade and is separated from the blade by about 0.030 inches ofelectrolyte. This electrode is formed from stainless steel. A voltage of45 volts is employed which produces a current density of about 0.5 ampsper square centimeter of blade area. A polishing cycle lasts for about30 seconds at which time pre-existing protective coating (PWA 47) isfound to be completely removed from the blade in the area adjacent tothe electrode. The voltage is then reduced to about 4 volts whichproduces a current density of about 0.03 amps per square centimeter foretching. Etching continues for about 10 to 15 seconds to produce asurface condition suitable for metallographic examinations. A plasticreplica is made of the etched surface and is examined. Examination byone skilled in the art will readily reveal if the balde has been exposedto excessive temperatures and will indicate if the blade is suitable forreuse. If the blade is suitable for reuse, the entire protective coatingmay be chemically stripped from the blade using any one of several wellknown solutions. The blade may then be cleaned by shot peening and thenrecoated with the same or another protective coating and reinstalled inthe engine.

An alternate solution which may be employed is one consisting of about13% concentrated sulfuric acid, balance methanol. The process of theinvention has been used to remove PWA 47 coating (a diffusedaluminum-silicon coating produced by vapor deposition) from IN100 enginecomponents having a nominal composition of 9.5 Cr, 15 Co, 3 Mo, 4.8 Ti,5.5 Al, 0.95 V, 0.17 C, 0.006 Zr, 0.015 B, balance Ni.

Although this invention has been shown and described with respect to apreferred embodiment thereof, it should be understood by those skilledin the art that various changes and mossions in the form and detailthereof may be made therein without departing from the spirit and scopeof the invention.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. A nondestructive method for examining gas turbine enginecomponents, of the type having a superalloy substrate and a protectivealuminide coating thereon, after service including the steps of:a.mechanically abrading the blade surface to remove any oxide coating; b.immersing the blade in an electrolytic solution in which an electrode isalso immersed; c. applying a voltage between the blade and theelectrode, through the electrolytic solution, so that the blade ispositive respect to the electrode; d. regulating the voltage to causeelectropolishing of the component until the aluminide coating isessentially removed to expose the substrate; e. changing the voltage bymeans external to the electrolyte-blade-electrode combination, to causeelectroetching of the underlying substrate; f. examining the etchedsubstrate, by optical or electron microscopy techniques to determine themetallographic structure and gamma prime morphology so as to determinethe condition of the substrate and its suitability for further use.
 2. Amethod as in claim 1 wherein the electrolytic solution contains aceticacid and perchloric acid.
 3. A method as in claim 1 wherein theelectrolytic solution contains sulfuric acid and methanol.